I take it you're in the city of Riverside itself rather than one of the outlying suburbs. The city probably will keep a list of historicals, but your chances of finding a recent sale of one of them is pretty slim. However, sales of homes from the '20s and '30s shouldn't be that scarce; there's a bunch of those properties in the core areas of the city. There's also a bunch of them in the city of San Bernardino or maybe Redlands. I know, I know, different county. But there is some economic correlation there between the two areas. As a last resort you could look and see.
The first thing to look out for is the zoning. A lot of times these homes are located in neighborhoods at or near the center of town, and their underlying sites might not have SFR zoning; they might be multi-family or commercial zoning. You wouldn't care if its multi-family, but if it's commercial zoning you might possibly be looking at a highest and best use issue in the form of the property being worth more as an office conversion potential than a residence. Given the recent trends in SFR pricing in SoCal, I kinda doubt any single tenant office use would be profitable enough to attempt such a conversion but you never know until you look. Remember, just having the legal right to convert doesn't automatically make it the HBU; it also has to be more profitable to do it that way instead of simply leaving it "as is" or rehabbing it as a residence.
One of the questions you'll need to resolve is what it would take to update the place so that it has reasonable plumbing, electrical. If the interior isn't relatively contemporary, you may have a floor heater or other form of heating that isn't real efficient. The kitchen and baths may need some attention, too. As an historical, significant alterations of the exterior design are probably out, but that's okay since the typical buyer is going to be a big fan of that kind of thing anyway. However, the interiors of these homes can usually be remodeled. Homes in that age range usually are of frame/siding construction so you'll be looking for signs of deterioration and termite infestation; check for a stable foundation, and at least take a look at the fusebox. Those items will usually tell you a lot about the structure.
The only other thing is to make sure the previous owners haven't deeded the exterior facade over to the historical society or the city. There are a few places in SoCal where they've done that, and in doing so have basically ceded all control over the facade to an outside entity. In the case of an activist historical society or city agency, this can even prevent the new owners from fixing things that need to be fixed without an extensive and costly review process. Sometimes there are property tax credits involved. It doesn't always affect the value or the marketability, but if present, it can.
If you don't ask the questions, they probably won't give you the answers. If you're not ready to ask the questions and then take all the steps necessary to address them in your appraisal then you should probably pass the assignment on to a more experienced appraiser. This property would probably classify as a complex assignment, so hopefully your supervisor is at least a Certified Res. If not, then he/she shouldn't be doing it, either. IMO.
